Sahih Muslim 2756
صحيح مسلم#2756SahihAbu Huraira (Allah be pleased with him) reported that he (the Messenger of Allah) was asked as to which prayer was most excellent after the prescribed prayer, and which fast was most excellent after the month of Ramadan. He said:Prayer offered in the middle of the night and the most excellent fast after (fasting) in the month of Ramadan is the fast in God's month al-Muharram
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Every narration in Sahih Muslim is accepted as Sahih (authentic) by the established status of the book.
In-book reference: 13:262